1. Field of the Invention
This invention relates to an improved coupling for connecting a pipe joint and a metal pipe which has a relatively small diameter not a wall thickness not exceeding about 2 mm, and which is often used to form a pipeline for supplying oil, air, etc. to an automobile, machine or apparatus, or collecting exhaust gas, etc. therefrom.
2. Description of the Prior Art
A known coupling of the type to which this invention pertains is shown by way of example in FIG. 24. A pipe joint 31 has an axial bore 32 which defines a fluid passageway. The bore 32 has a diametrically enlarged portion 35 defined by an annular shoulder formed on the inner wall surface of the joint 31. A metal pipe P.sub.1 has an end portion fitted in the enlarged portion 35. The end portion of the pipe P.sub.1 ' has an outer wall surface welded to the inner wall surface of the joint 31 as shown at W. They are welded manually by means of a burner, or are welded in a heating furnace.
The welding work which is employed for connecting the pipe and the joint, however, presents a number of problems. It is only after the welding work is finished that the pipe and the joint can be electroplated with zinc, a zinc alloy, etc. to obtain the necessary corrosion resistance. The electroplating job complicates the process of pipeline connection and gives rise to a reduction in productivity. It is difficult to form an electroplated layer having a uniform thickness, as it is impossible to maintain a constant distance between the poles and a constant current density throughout the electroplating work. An unduly long time is required for making uniform the thickness of the electroplated layer. The welding of the pipe to the joint often results in localized heating, or overheating. The overheating of the pipe lowers its mechanical strength and the vibration of an engine or machine to which the pipe is connected causes it to crack or break adjacent to its portion welded to the joint.